1. Field of the invention
This invention relates to chain-type conveyors that are used for conveying sheet materials, and more particularly to a guide rail structure for guiding the movement of a conveyor chain along a predetermined path and to protect the chain from jamming as a result of an accumulation of chips of the material being conveyed and other particulate matter, as well as to guide the chain in a predetermined direction and with a minimum of frictional drag.
2. Description of the related art
Chain-type conveyors for conveying sheet materials, such as sheets of plastic, oftentimes traverse paths that subject the chain to chips of plastic or other particulate matter that can accumulate on the chain or on the chain guide rail. Such accumulations of material can cause stopping or slowing of the movement of the chain and thereby impede or interrupt the process to which the sheet is intended to be subjected.
For example, in thermoforming apparatus in which a sheet of thermoplastic material is carried through a heating device to raise the temperature of the plastic material, and thereby soften it so that it can be drawn by vacuum into a plurality of molds to form containers and other drawn parts, the sheet is generally carried by a pair of laterally spaced conveyor chains that are parallel to each other. The chains each carry a series of piercing spikes to pierce the sheet adjacent to its outer edges so that the sheet can be positively conveyed through the machine. Oftentimes the areas in which thermoforming machines are located include scrap grinding apparatus that grinds scrap plastic into small chips, some of which may fall onto the chain or be carried by the softened edges of the plastic sheet, and when such particles have accumulated on the chain or chain guide rail they can cause a jam and slow or stop the movement of the chain.
In addition to the possible jamming of such a conveyor chain by chips and other debris, chain conveyors in thermoforming machines carry sheets of plastic through heating devices that serve to raise the temperature of the sheets so that it can be vacuum drawn into a plural cavity mold to form containers and other similar parts. In the course of carrying the sheet through the heating device, the sheet expands in a transverse direction, as a result of thermal expansion, and oftentimes the chain guide rail structures are such that insufficient space is provided to permit such lateral expansion, thereby also contributing to jams caused by interference between the edges of the sheet being conveyed and the chain guide rail structure.
Another problem experienced in guide structures for conveyor chains is excessive frictional drag when the chain is carried on a track that includes a lower track on which the chain rollers ride, and an upper track that is positioned adjacent to the upwardly facing surfaces of the chain roller. In such an arrangement the inner surface of the links that connect the several chain rollers slide against the sides of the upper and lower tracks, thereby imposing frictional drag that requires additional power to drive the chain at the proper speed, and also causing excessive wear of the contacting parts that requires periodic removal and replacement of either entire chains or of individual chain links.
It is an object of the present invention to overcome the problems that have been pointed out above in connection with prior art conveyor chain guide structures.
It is another object of the present invention to provide an improved guide apparatus for guiding a conveyor chain and to protect the chain from chips and other foreign particulate matter.
It is still another object of the present invention to provide a guide apparatus for guiding a conveyor chain and in which adequate space is provided for lateral expansion of a sheet being conveyed without interference between the edges of the sheet and the chain guide structure.